A large number of electrical devices as well as electrical lines are protected by fuses in the event of a fault. Faults of an extremely wide variety of types can occur. The most common faults can be understood as overload faults or as short-circuit faults.
Typically, a fuse can then be tripped. The current flowing through the fuse heats the fuse element to the extent that at least partial, if not complete, fusion of the fuse element occurs. As a rule, this fusion is associated with the occurrence of an electric arc, in which case material of the fuse element vaporizes. This vapor precipitates in another location, and the electric arc is cooled to the point that the current is limited and finally shut off.
The fusion of the fuse element is determined by its material and geometric characteristics, so that, depending on the material and/or geometry of the fuse element, a respective heat quantity Q is required to vaporize the fuse element. Typically, the fusing characteristics and rated tripping currents associated therewith are described by the melting integral I2t.
It should be borne in mind, however, that this current, which represents a fault condition, still flows through the device or system to be protected.
Particularly in the case of high short-circuit currents, the danger therefore exists of damage occurring that should actually be prevented, since the power limit of the device to be protected is exceeded.
In addition, it must be considered that current is flowing not only in the fusing phase of the fuse element, but also in the quenching phase.
That is, only the integration of the two current flow ranges over time results in the pass integral.
Thus, in the process of dimensioning, it is actually this pass integral that must be considered in order to avoid damage.
However, this is often erroneously neglected, thus resulting in incorrect dimensioning.
Special requirements apply if the device to be protected is an overvoltage protection device; after all, these are supposed to temporarily allow high currents to pass through without tripping the fuse, yet switch off promptly even in the event of low, lasting fault currents such as those which can occur if the overvoltage protection device is damaged, or as secondary current, for example. While the first requirement often leads to high rated current values for the fuse, the second requirement can be sensibly met only with low rated current values.
At the same time, there has been an ever-stronger trend toward small installation spaces. The requirements therefore cannot be met with existing fuses.